신비한사전

Somevit Island, floating at the southern end of Banpo Bridge, has no pillars sunk into the riverbed. The Han River's water level rises and falls so much that its banks can be completely submerged during heavy rains, and pillars would create obstacles blocking the water flow. Therefore, they chose a method of mooring it while allowing it to move up and down. Concrete blocks buried in the riverbed and thick chains hold it in place laterally, and as the water level rises, the island rises with it. This was the world's first time a building was entirely placed on a floating platform.

In the 1960s, Seoul had no sewage system, so they just dumped wastewater into the neighborhood streams. The smell, mosquitoes, and overflowing water when it rained. Streams were a nuisance back then. So, they covered them with concrete. Covering them eliminated the smell and the surface became a road. The problem is that the covered streams became not streams, but massive sewer pipes. However, they couldn't just uncover all of them. Roads and shops were built on top, and even if you dug them up, there was no water inside. The water flowing in Cheonggyecheon now is drawn from the Han River and the subway.

Over a four-year period starting in 1982, 63.69 million cubic meters of sand and gravel disappeared from the bottom of the Han River. That's enough to fill a 500m x 500m area to the height of the 63 Building. It had to be dredged to stabilize the river channel, but there was nowhere to dump it and no money to transport it. So, it wasn't seen as waste soil but as a commodity to be sold. Salt-free river sand is highly valued in concrete, and at that time, Seoul was in the midst of a massive apartment construction boom. The 196.2 billion won earned from selling the aggregate covered one-fifth of the total construction costs.

The amount of rain Seoul can handle is set at 100mm per hour. This number isn't just a guess; it's a probability derived from rainfall records accumulated at observation stations nationwide. It's calculated based on rainfall that occurs once every 30 years, with a little extra buffer added to set the current standard. The problem is that this number is both a promise and a declaration of limits. In the summer of 2022, Dongjak-gu experienced 141mm of rain per hour, when Seoul's standard was 95mm. Therefore, instead of grouping the entire city under one number, Seoul has opted to set different standards for each neighborhood and channel excess rain into deep underground tunnels.

Even in winter, the Han River doesn't reveal its bottom these days. But 40 years ago, during the dry season, sandbars would prominently appear, and during the rainy season, it would flood over the banks. It all started in 1982 when they dredged a 36km stretch from Amsa-dong to Haengju Bridge, fixing the waterway to a depth of 2.5m. However, digging deeper caused the water to drain even faster, and saltwater flowed back upstream during high tide. The solution wasn't to dam the estuary, but to build underwater steps. A submerged low weir beneath Jamsil Bridge is what currently holds the Han River's water level.

Jeju Island receives nearly 4 billion tons of rain annually. However, in the past, Jeju residents had to carry water and walk for hours. This was because the porous basalt of the volcanic island was riddled with holes, causing rain to seep directly into the ground as soon as it fell. As a result, villages were all lined up in a circle along the coast, where the water that seeped underground would resurface upon meeting the sea. Nowadays, they drill wells to pump water directly, and it's said that this water is naturally filtered as it passes through the volcanic rock layers for decades.

Seoul has six plants that pump up Han River water to turn it into tap water. They produce 3.8 million tons daily, enough for Seoul's 10 million residents and four cities in Gyeonggi Province. How does murky river water actually become drinkable? The key wasn't forcing out tiny particles, but clumping them together to make them bigger and then letting them settle. However, the smell from summer algal blooms dissolves in the water and can't be filtered by sand. So, they added two more stages using ozone and charcoal.

Roads that were fine one day suddenly develop holes the size of a pot overnight. In Korea alone, over 500,000 of these holes are created and filled each year. Why do roads that can withstand trucks weighing tens of tons suddenly develop these issues, especially after rain? The culprit is water. Water seeps through cracks in the asphalt and freezes on winter nights, pushing the road apart from the inside. That's why nowadays, instead of blocking water, they use porous pavement that allows water to pass through and drain. Apparently, this is also why less water splashes up on rainy roads.

There's a place where all the water that Seoulites use and discard throughout the day is collected. It's at four water reclamation centers. They purify the black wastewater so thoroughly that it can be released into the Han River. The secret wasn't filtering, but rather having microorganisms consume it. Most of the dirt in wastewater is dissolved, so it can't be filtered out with a sieve. Therefore, they inject air to cultivate a large number of microorganisms, which then consume the organic matter. Nowadays, they're apparently burying these facilities entirely underground and covering the top with parks.

In 2019, when people in Incheon turned on their taps, murky, reddish-brown water poured out. It wasn't because a water treatment plant had malfunctioned or the river water had become dirty. It all started with a routine operation: temporarily shutting down one water treatment plant and rerouting water from another. The problem was that the water flowed in the opposite direction of normal. This dislodged decades' worth of sediment that had accumulated inside the water pipes. A staggering 260,000 households had to rely on bottled water for over two months, and apparently, the entire water system transfer procedure was changed afterward.